A so-called part-time four-wheel-drive vehicle is comprised of primary axles (front axles for example) steadily driven by a primary drivetrain and secondary axles (rear axles for example) driven by a secondary drivetrain. When a changeover device uncouples the secondary drivetrain from the primary drivetrain, the vehicle runs in a two-wheel-drive mode. When the changeover device couples the secondary drivetrain with the primary drivetrain, drive force is transmitted via a shaft to the secondary drivetrain, thereby realizing a four-wheel-drive mode.
Although the aforementioned changeover is, without a speed synchronizer, enabled only in a case where the vehicle is idling, there are also known drivetrains which enable changeover during travel. In the latter drivetrains, frictionally-locking clutches are often applied.
Japanese Patent Application Laid-open No. 2009-269605, US Publication 2010/0094519, and Japanese Patent Application Laid-open No. 2003-335143 disclose related arts.